1. Field of the Invention
The present invention relates to an image forming apparatus including a transfer device, and more particularly relates to an image forming apparatus which enhances a transfer of an image with the transfer device.
2. Discussion of the Background
An image forming apparatus (e.g., a copier, facsimile, printer or similar image forming apparatus) is known to use a transfer system capable of transferring an image to a recording medium, e.g. a paper sheet, or which utilizes an intermediate transfer system. Such transfer systems have a transfer device, which contacts an image carrier, for transferring a toner image from the image carrier to the recording medium by applying a transfer bias voltage.
However, when utilizing such a transfer system it is possible that the transfer bias voltage may be improperly discharged (e.g., a leakage may occur) if the transfer device or the image carrier has pinholes, defects, or the like. Further, it is possible that the recording medium may be improperly transferred or an image forming may be improperly operated because of noise generated from the improper discharge.
Preventing such an improper discharge has been addressed. For example, Japanese Laid-Open Patent Publication No. 7-13440 discloses a transfer device having a transfer belt and plural electrodes for applying a transfer bias to the transfer belt. Japanese Laid-Open Patent Publication No. 8-152789 discloses a transfer device having a transfer belt and a transfer bias electrode that has at least a two-layer structure in order to prevent an improper discharge from the transfer belt to an image carrier. Japanese Laid-Open Patent Publication No. 9-73239 discloses a relation between a maximum applied voltage to an electrode and a shortest distance between a surface of the electrode and an image carrier in order to prevent an improper discharge from the transfer belt to the image carrier.
However, each of the solutions disclosed in the above-noted publications can only provide a limited solution to preventing the improper discharge. This is particularly the case since in recent years high-speed image forming apparatuses are starting to use a transfer system. In such high-speed image forming apparatuses, the amount of discharge for a unit area to be deposited on the transfer belt may be required to be increased if a process speed, including a moving speed of a transfer belt for forming a toner image, increases due to speeding up of the image forming operation, and as an applied voltage to the transfer belt increases. Thus, in such high-speed image forming apparatuses it becomes more likely that a transfer bias voltage may be improperly discharged (e.g., a leakage may occur). The solutions noted in the above background art may be inadequate to prevent an improper discharge under such conditions.